In an image forming apparatus, such as a dry electrophotographic image forming apparatus, an unfixed toner image is formed on a recording medium, and subsequently this toner image is fixed thereon to form an image. In the image forming apparatus as described above, as means for fixing an unfixed toner image which adheres to a recording medium, in general, a device has been used in which a recording medium carrying an unfixed toner image thereon is allowed to pass between a heating roller and a pressure roller so as to fix a toner on the recording medium by heat and pressure.
However, according to the fixing means described above, electrical power consumption used to maintain the heating roller at a toner fixable temperature is high, and on the other hand, when the heating roller is placed in a stand-by state at a lower temperature in order to save the electrical power, the time required for the start-up disadvantageously increases. In addition, since the recording medium is supplied between the heating roller and the pressure roller, paper jam may also disadvantageously occur.
In order to overcome the problems described above, a fixing method to melt-fix a toner itself in a non-contact manner using induction heating has been proposed in which a toner containing metal particles and/or magnetic particles which generates heat by high-frequency magnetic induction is used, and in which a recording medium carrying an unfixed toner image thereon is allowed to pass in a high-frequency magnetic field (Patent document 1).
According to the method described above, as the heat generation mechanism, either: a heat generation phenomenon is used in which when a high-frequency magnetic field is applied to a toner containing fine metal particles, heat is generated by eddy-current losses caused by eddy currents flowing in the fine metal particles; or a heat generation phenomenon is used in which when a high-frequency magnetic field is applied to a toner containing ferromagnetic fine particles, heat is generated by hysteresis losses caused by magnetic hysteresis characteristics of the ferromagnetic fine particles.
However, in the case in which a toner containing magnetic particles is used in the method described above, when an unfixed toner image passes under a magnetic field, toner particles are scattered by the influence of magnetic flux lines, and as a result, a problem in that an image is distorted may arise in some cases.
In addition, in a high-frequency magnetic induction heating method using the eddy-current losses or the hysteresis losses described above, in order to generate heat required to melt-fix a toner, magnetic particles contained in the toner must have a relatively large particle diameter (approximately one micrometer to several tens of micrometers).
That is, in an eddy-current loss method, since the eddy-current loss P is proportional to the square of the product of a fine metal particle diameter d, an excitation frequency f, and a magnetic flux density B, in order to ensure the current loss P required to melt-fix a toner, the particle diameter d must be increased to a certain extent. In addition, in a hysteresis loss method, as the particle diameter of ferromagnetic fine particles is decreased, a loop area of the magnetic hysteresis decreases. When the particle diameter is decreased to less than 100 nm, the magnetic hysteresis characteristics totally disappear, and the fine particles have superparamagnetic characteristics. Accordingly, heating cannot be performed by the magnetic hysteresis loss, and hence, the particle diameter cannot be decreased.
Accordingly, in a toner using these fine metal particles, since the metal strongly absorbs visible light, the transparency of the toner is degraded, and hence a problem in that the toner can only be used to form a monochromatic image may also arise.
Through intensive research carried out by the inventors of the present invention on the behavior of magnetic particles under the influence of a magnetic filed, it was discovered that magnetic particles are scattered because the magnetic particles are magnetized by a high-frequency magnetic field applied thereto from the outside, and therefore: magnetic particles adjacent in a magnetic flux line direction are attracted and/or aggregated to each other; or magnetic particles present in a direction perpendicular to the magnetic flux line direction are repelled to each other, so that positions of the magnetic particles are considerably changed. Furthermore, since the magnetic force generated by magnetization is inversely proportional to the square of the distance between particles, once the aggregation thereof starts, an avalanche phenomenon occurs, and as a result, the aggregation occurs over the entire region to which a magnetic field is applied.
Incidentally, the intensity of magnetic force generated by magnetization approximately depends on the mass of a magnetic particle. Hence, when magnetic particles having a high heat-generation efficiency (quantity of heat generated per unit mass of a magnetic substance) are used, and the amount of magnetic particles contained in a toner can be decreased, the above magnetic force acting on each toner particle containing the magnetic particles can be decreased, and hence it is believed that scattering of toner can be prevented.
Accordingly, alternative magnetic particles having a high heat-generation efficiency which generate heat by high-frequency magnetic induction were investigated, and it was found that heat-generation phenomena based on Neel relaxation and Brown relaxation observed in superparamagnetic fine particles having a particle diameter of less than 100 nm have a significantly high heat-generation efficiency, and that superparamagnetic fine particles having a particle diameter of less than 100 nm are suitably used as heat-generation magnetic particles for a heat fixing toner.
In recent researches on the characteristics of superparamagnetic fine particles having a particle diameter of less than 100 nm, a heat-generation phenomenon based on the principle called Brown relaxation and/or Neel relaxation has been observed under high-frequency magnetic field excitation. Brown relaxation is a relaxation phenomenon which occurs in such a way that when a high-frequency magnetic field is applied to superparamagnetic particles, the particles themselves mechanically rotate in order to conform their magnetization direction to the change between the positive and negative directions of the high-frequency magnetic field. Since the rotation described above cannot follow the change of high-frequency magnetic field due to the friction between the particles and a liquid, hysteresis of the magnetic field versus the magnetization is generated, so that heat is generated. This is the heat-generation caused by Brown relaxation. On the other hand, Neel relaxation occurs in such a way that when a high-frequency magnetic field is applied to superparamagnetic particles, the magnetization direction rotates in order to conform their magnetization direction to the change between the positive and negative directions of the high-frequency magnetic field. Since the superparamagnetic particles have a small particle volume, their magnetization produce a heat fluctuation, thus cannot follow the change of high-frequency magnetic field. As a result, the hysteresis is generated, thereby causing a heat generation. In addition, heat generation caused by a so-called hysteresis loss is an exothermic phenomenon caused by a hysteresis itself which is generated when a high-frequency magnetic field is applied to a magnetic substance having a multi-magnetic domain structure.
In addition, through a research carried out by the inventors of the present invention on the relationship between a particle diameter d and a heat release value P, it was found that in superparamagnetic ferrite fine particles having a particle diameter of 18 to 23 nm, heat generation caused by Neel relaxation is particularly significant, and a superior heat-generation efficiency can be obtained as compared to that of superparamagnetic ferrite fine particles having a different particle diameter.
Based on the findings described above, the inventors of the present invention carried out high-frequency magnetic induction heating by Neel relaxation and/or brown relaxation using a compound which contains superparamagnetic fine particles having a particle diameter of less than 100 nm as a toner, and as a result, it was found that in an image forming method in which an unfixed toner image is melt-fixed in a non-contact manner by high-frequency magnetic induction heating, the toner particles are not scattered, and a non-distorted high quality image can be formed.